Wireless communication systems available in recent years are constructed to achieve a throughput of about several hundreds Mbps based on the background technology of MIMO (Multiple Input Multiple Output) multiplex transmission. The MIMO multiplex transmission can employ multiple code word transmission for sending a large quantity of bits using the same frequency and time resource at the same time. A decoding process for the multiple code word transmission may be realized by as many decoders as the number of code words.
FIG. 1 is a diagram showing an example of a receiver using a plurality of decoders.
As shown in FIG. 1, the receiver includes channel estimator 110 for estimating channels using received signals of pilot symbols from respective transmission antennas, SNR estimator 120 for calculating SNRs (Signal to Noise Ratios) at respective reception antennas from the channels of the pilot symbols estimated by channel estimator 110, MIMO demodulator 130 for separating signals sent by way of MIMO multiplex transmission, decoding processor 150 for decoding code words from MIMO demodulator 130 using the LLR (Log Likelihood Ratio) of the code words, CRC checker 160 for performing a CRC check on estimated code words that are produced from decoding processor 150 which decodes the code words, and ACK/NAK signal generator 170 for generating information to be fed back to the transmitter. Decoding processor 150 includes turbo decoding engine 151 having as many turbo decoders 151-1, 151-2 as the number of the code words.
As many turbo decoders as the number of the code words make it possible to decode a large quantity of bits that have been sent using the same frequency and time resource at the same time.
JP No. 2007-6382A discloses a method of determining a maximum iteration count of each code block based on an allowed time that can be allocated to the iterative decoding in one frame, the number of code blocks in one frame, and the size of each code block.
The receiver which has as many iterative decoders as the number of code words, as described above, requires as many iterative decoders as the number of code words. If the number of code words is large, then the receiver needs to have a large circuit scale and finds it difficult to reduce the entire size thereof.
If different modulation schemes and different error-correcting coding ratios are applied to respective code words in the MIMO multiplex transmission, then since TBSs (Transport Block Sizes) are different, the iterative decoders have different processing times.
FIG. 2 is a diagram showing an example of decoding processing times at the time respective code words have different TBSs in turbo decoding engine 151 shown in FIG. 1.
As shown in FIG. 2, even if the processing of a code word with a small TBS is finished early, since feedback information is sent to the transmitter after ACK/NACK signals of all code words have been generated, the ACK/NACK signals to be fed back to the trans-mitter are governed by the processing time of a code word having a large TBS. Therefore, the decoding processor has a redundant configuration.
The technology disclosed in JP No. 2007-6382A alone is unable to process each of the received code words, and fails to solve the problems that arise if different modulation schemes and different error-correcting coding radios are applied to respective code words in the MIMO multiplex transmission.